1. Field of the Invention
The present invention relates to a method of thermally treating semiconductor wafers in a furnace and a tool for holding the semiconductor wafers in the furnace. The present invention also relates to a semiconductor wafer having a shape suitable for the thermal treatment of the semiconductor wafer.
2. Description of Background Arts
As well known in the field of semiconductor technology, semiconductor devices are obtained by fabricating electronic elements and wiring structures on a semiconductor wafer and then splitting the semiconductor wafers into chips. In fabricating the various electronic structures on the semiconductor wafer, the wafer is thermally treated in a furnace for impurity diffusion, thermal oxidation or the like.
FIG. 19 illustrates a wafer boat 2 holding semiconductor wafers 1 thereon, and a sectional view taken along a line II--II is illustrated in FIG. 20. The boat 2 is a block of quartz or polysilicon having a shape of a shallow gutter, and a periodic array of grooves 3 are formed thereon. Respective bottom edges of the wafers 1 are inserted into the grooves 3, whereby the large number of the standing wafers 1 are supported by the boat 2. The boat 2 has chamfers 5 at respective bottom edges thereof, which fit to the inner surface 4 of a heater tube in the furnace.
In the case where the wafers 1 held with the boat 2 are subjected to a heat treatment in the furnace, the following problems are caused.
One of the problems is plastic deformation of the wafers 1 due to the gravity thereof. As shown in FIG. 21 which is a partial enlarged view of FIG. 20, the width of the grooves 3 is set at a value larger than the thickness of the wafers 1 so that the inner walls of the grooves 3 may not compress the wafers 1 when the wafers 1 and the boat 2 are thermally expanded in the furnace. Accordingly, the wafers 1 stand on the boat 2 while being somewhat inclined due to the gap or clearance between the wafers 1 and the inner walls of the grooves 3. When the heat treatment of the wafers 1 held by the boat 2 is conducted for a long time, plastic deformation of the wafers 1 is caused by the gravity thereof. Therefore, the wafers 1 subjected to the heat treatment become deformated ones, and accuracy in work and positional adjustment of a mask is decreased in the following processes such as lapping, epitaxial growth and photolithograghy. As a result, the fraction defective in semiconductor devices obtained from the wafers 1 is increased.
In particular, the plastic deformation is relatively large in wafers having large diameters, and the fraction defective becomes large in proportion to the deformation. Furthermore, even if the diameters of wafers are not so large, the plastic deformation becomes relatively large under the condition that the thickness of the wafers is small.
Another problem of the prior art relates to a holding position of the wafers 1. Since the boat 2 is placed at a position close to the inner bottom surface of the heater tube, the center points CW of the wafers 1 (FIG. 19) are inevitably located at a position which is considerably lower than the center point CT of the inner space of the furnace. In the case where the heat treatment of the wafers 1 in the furnace is conducted for diffusing impurities into the wafers 1, residual impurities and dusts are often deposited on the inner bottom surface of the heater tube. The residual impurities and the dusts go up into the air due to the air current caused by frictional movement of the boat 2 on the bottom surface, when the boat 2 holding the wafers 1 is put into the heater tube. Consequently, the residual impurities and the dusts randomly adhere to the surfaces of the wafers 1 which are held at the low positions. When the wafers 1 are subjected to the heat treatment, non-uniformity of heat treatment such as deviation in the density distribution of impurities and an abnormal diffusion is caused by the adhered residual impurities and dusts. As a result, the electric character of the wafers 1 is deteriorated and the fraction defective thereof is increased.
FIG. 22 illustrates another conventional holder 6 for holding the wafers 1 in a furnace. The holder 6 is constructed by connecting four rod members having notches 7 with each other, and the wafers 1 are held in the holder 6 with respective edges being inserted into the notches 7. In further another holder, which is not illustrated in the drawings, the wafers 1 are held by the holder in the state where the wafers 1 are stacked in the vertical direction with spaces therebetween. Respective major surfaces of the wafers 1 are in horizontal planes and respective edges of the wafers 1 are inserted into notches formed in the holder.
Although the constructions of these holders are different from that of the boat 2 shown in FIG. 19 and FIG. 20, the above-indicated problems still remain since plastic deformation is caused in the wafers 1 and the holding positions of the wafers 1 are close to the bottom surface of a heater tube.